1. Field of the Invention
The present invention relates to heated air data sensors and in particular to sensors which use PTC resistance material for such heaters.
2. Prior Art
In the prior art the use of positive temperature coefficient resistance material for heating various devices has been disclosed. British Patent Specification No. 1,371,709 shows a pressure head for air data sensors having a heater element or elements made of a PTC resistance material. As disclosed the material has a low level electrical resistance until it reaches a given temperature called the anomaly (or Curie) temperature and then the resistance rises substantially when that temperature is reached. The pressure head is de-iced with toroidal or tubular ring type heaters mounted inside the pressure head with the outer peripheral surfaces of the heaters against the inside wall of the body of the pressure head. Difficulties are encountered in this type of device from the manufacturing of the heater elements in the proper shape and the close tolerance required, and also there are problems of installation which limit the effectiveness of the thermal conduction between the heater and the wall of the probe.
Other applications of such PTC resistor heaters include the use in a windshield wiper blade, shown in U.S. Pat. No. 3,489,884, and also as a heating device for use with aerosol containers as shown in U.S. Pat. No. 3,338,476. A fully automatic coffee pot using a PTC resistor heater is shown in U.S. Pat. No. 3,375,774.
One important PTC material used at the present time is doped barium titanate, a material whose positive temperature coefficient properties have long been recognized. Any heater installation in an air data sensing vane must be in good thermal contact with the vane and at the same time electrically insulated from the vane. Various encapsulating materials have been used with PTC elements. For example, U.S. Pat. No. 3,824,328 indicates that a capsulation material can include silicone resins, polyamides and polyimides, and then external epoxy can be used for potting on the outside of the initial layer. A PTC resistor package is also shown in U.S. Pat. No. 3,835,434. Bonding of heating elements to a structure is illustrated in U.S. Pat. No. 3,898,422.
PTC materials have been used in other applications, such as motor starting apparatus, degaussing systems for color television tubes, and also in various current limiting applications.